Stable supply of various chemical products using petroleum as a source material has been getting difficult recently due to worldwide depletion of petroleum resources and price hike derived therefrom. In 1980s the international price of naphtha was approximately 20 dollars per barrel, however in 2010s it is approaching 100 dollars per barrel. As the result, a price increase and profitability deterioration of plastics using petroleum as a source material have come to occur actually. Especially, in Japan, which is lacking in stably available natural resources, an influence of petroleum price hike is serious, and a proposal from a research directed to a structural innovation of chemical industries is required urgently.
Under such circumstances, there has been a great expectation in effective utilization of biomass which resources of biological origin. However, for effective utilization of lignocellulose (biomass), which is a mixture of lignin, hemicellulose, and cellulose, a separation step for the respective ingredients is currently indispensable. Ordinarily, either of a polysaccharide (cellulose and hemicellulose) or a lignin is selectively degraded, and one ingredient in the biomass is recovered as a low-molecular-weight compound and the other ingredient in the biomass is recovered as a polymer. In general, after the above purification step, the intermediate products are converted in an organic chemical manner or a biochemical manner to a useful chemical substance. For example, in a case where cellulose acetate is produced using biomass as a source material, cellulose acetate with a desired acetylation degree is produced from wood chips as a source material by producing a high purity wood pulp or cotton linter (main ingredient is cellulose) through a digestion step, a cleaning and washing step, an enzymatic delignification step, a bleaching step, etc.; activating the wood pulp or cotton linter by a pretreatment, performing a esterification reaction in an acetylation step by adding acetic anhydride, and acetic acid, as well as sulfuric acid as a catalyst to the cellulose, and implementing an aging step. However, the above production process is economically and thermally disadvantageous, and therefore not yet able to replace an existing petrochemical process. Further, the degree of polymerization of cellulose is decreased during production of wood pulp, and cellulose acetate obtained therefrom has only a low degree of polymerization, and as the result its mechanical property becomes lower and handling becomes difficult when the same is processed to a fiber or a film, etc. and moreover there arises a drawback that the color tone of a processed product becomes yellowish due to thermal degradation, etc. during processing.
Meanwhile, lignin is a polymer composed of an aromatic compound and one of the main ingredients composing a plant cell wall together with polysaccharides (cellulose and hemicellulose). Lignin is obtained as a byproduct of a paper pulp production process or a bio-ethanol production process, however it is only utilized mainly as a fuel, and its industrial utilization has not yet advanced well in the present circumstances.
Recently, utilization of an ionic liquid has been proposed for a treatment of biomass. However, in many of preceding studies reported so far, an ionic liquid is used in a pretreatment step for biomass for the purpose of partial relaxation of a biomass structure. Therefore, a subsequent degradation reaction such as degradation of a polysaccharide ingredient by an enzymatic saccharification reaction is indispensable. Consequently, this technology is heretofore economically disadvantageous compared to existing technologies. A technology to easily synthesize and separate a polysaccharide derivative such as a polysaccharide ester and a lignin derivative such as a lignin ester directly from biomass is believed to be thermodynamically and economically advantageous, however it has been hardly achievable.
Meanwhile, some technologies for derivatizing cellulose by utilizing an ionic liquid have been known. Since an ionic liquid has an extremely low volatility, and therefore is free from risks of contamination or ignition by volatilization, and further it has a high dissolving power with respect to cellulose, research and development works thereof as a solvent for processing cellulose have been in progress. For example, Non Patent Literature 1 discloses a method by which cellulose is esterified using an ionic liquid of 1-butyl-3-methylimidazolium chloride (BMIMCl) as a solvent, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst.
Further, Patent Literature 1 discloses a method by which cellulose is swelled with acetic acid, acetic anhydride and sulfuric acid are added thereto to react so as to produce cellulose acetate (cellulose derivative), and in a mixture containing the cellulose derivative, an ionic liquid such as 1-butyl-3-methylimidazolium chloride, and an esterification agent selected from the group consisting of a carboxylic anhydride, a carboxylic halide, and a carboxylic acid, the cellulose derivative is esterified to produce an esterified product of cellulose derivative.
The technology of Non Patent Literature 1 as described above is disadvantageous, because a catalyst is necessary separately in addition to an ionic liquid. Further, since the technology of Patent Literature 1 as described above uses sulfuric acid as a catalyst, there remains a drawback in terms of a waste treatment requirement, and also a problem of decrease in a molecular weight due to the use of a strong acid. Furthermore, since a carboxylic anhydride or the like is used as an esterification agent according to Patent Literature 1 and a carboxylic anhydride is corrosive, the technology is disadvantageous from the viewpoint of process.